Hydrogen is a very important material in the chemical industry. For example, it is used to produce ammonia and to synthesize hydrogen chloride. Also, it is an essential material for the hydrogenation in which unsaturated compounds are converted into saturated ones. In addition, hydrogen plays a pivotal role in improving the quality of petroleum products, that is, in the hydrotreating processes including hydrogen addition, desulfurization, denitrogenation and demetallization. Another example of the use of hydrogen is the contact hydrogenation of carbon dioxide in which carbon dioxide, which causes global warming, is reclaimed, immobilized and reused. In addition, hydrogen is viewed as a pollution-free, clear energy source substituting for existing fossil fuels.
Conventional techniques for obtaining hydrogen include extraction from fossil fuels, such as naphtha, modification of natural gas, reaction of vapor with iron, reaction of water with metal, electrolysis of water and etc. These techniques are economically unfavorable because immense heat or electric energy is required. The conventional techniques have another disadvantage of generating a large quantity of by-products, carbon dioxide or oxygen (upon electrolysis). As mentioned above, carbon dioxide is a serious air pollutant causing the greenhouse effect of the globe. When oxygen is generated, the hydrogen is difficult to separate from the oxygen, owing to which the process becomes complicated. Anyway, such by-products could make it difficult to obtain high purity hydrogen in high yields. Since the hydrogen production using such conventional techniques is usually carried out at relatively high temperatures, most of the related equipments, e.g. reactors and purifying apparatuses, are designed to be resistant to heat and thus, they are very expensive, which is an economically unfavorable factor.
Hydrogen gas can readily escape from the gravity of the earth because it is of low specific gravity and most of it exists in water or inorganic forms. For these reasons, only a small quantity of hydrogen exists in the atmosphere. Therefore, the development of the techniques whereby high purity hydrogen can be efficiently obtained from water is very important in that the urgent problem of exploiting substitute energy sources can be solved and the material for chemical industry can be secured.
Recently hydrogen producing techniques have been developed in which a photocatalyst is used to decompose water into hydrogen and oxygen. However, the prior arts relating to the photocatalyst for producing hydrogen are few published, representatives of which are exemplified by Japanese Pat. Laid-Open Publication Nos. Sho. 62-191045 and Sho. 63-107815.
Japanese Pat. Laid-Open Publication No. Sho. 62-191045 discloses that hydrogen is generated from an aqueous Na.sub.2 S solution in the presence of a rare-earth element compound by a photolysis reaction. The rare-earth element compound has an advantage of exhibiting an optical catalytic activity in the range of the visible light.
Japanese Pat. Laid-Open Publication No. Sho. 63-107815 describes a photolysis reaction in which a composite oxide of niobium and alkali earth metal is used as a photocatalyst, to generate hydrogen from a methanol solution in water. Likely, this photocatalyst has an advantage of being active in the range of the visible light.
However, these above prior arts are disadvantageous in that the amount of the hydrogen generated is very small.
Korean Pat. Appl'n. No. 95-7721, under the present inventor, which is believed to solve the above problems to some degree, suggests a photocatalyst represented by the following general formula I: EQU Cs(a)/K.sub.4 Nb.sub.6 O.sub.17 I
In the presence of the photocatalyst, ultraviolet light is irradiated onto an aqueous solution mixed with oxygen-containing organic compounds, such as formaldehyde and alcohol, acting as a hydrogen-generating promoter, to produce hydrogen.
This technique little affects the environment and can generate hydrogen at low temperatures, e.g. room temperature. However, the oxygen-containing organic compound as a hydrogen-generating promotor, makes it impossible to reuse the reactants.